With the technology development and the increasing demands for mobile devices, the demand for secondary batteries are also dramatically increasing, and particularly, lithium secondary batteries having a high energy density and a high operating voltage and good storage and life characteristics are being widely used as an energy source of various types of mobile devices as well as a variety of electronic products.
Secondary batteries are greatly classified into cylindrical batteries, prismatic batteries, and pouch type batteries according to external and internal structural features, and among them, prismatic batteries and pouch type batteries are particularly gaining attention due to being stackable at a high degree of integration and having a small width relative to a length.
An electrode assembly of a positive electrode/separator/negative electrode structure constituting a secondary battery is largely sorted into a jelly-roll type (winding type) and a stack type according to its structure. The jelly-roll type electrode assembly is made by coating an electrode active material on a metal foil used as a current collector, drying and pressing, tailoring in the shape of a band of desired width and length, interposing a separator between a negative electrode and a positive electrode, and rolling in a spiral pattern. The jelly-roll type electrode assembly is suitable for cylindrical batteries, but in the applications to prismatic or pouch type batteries, there are drawbacks such as a release problem of an electrode active material and low spatial utilization. On the other hand, the stack type electrode assembly has a structure in which a plurality of positive and negative electrode unit cells is stacked in a sequential order, and possesses an advantage of being easy to obtain a prismatic shape, but has disadvantages of a complex manufacturing process and a risk of a short circuit caused by misalignment of an electrode when impacts are applied.
To solve the problem, attempts have been made to develop an electrode assembly of a combined type of a jelly-roll type and a stack type, called a stack-folding type electrode assembly, having a structure in which a full cell of a positive electrode/separator/negative electrode structure or a bicell of a positive electrode (negative electrode)/separator/negative electrode (positive electrode)/separator/positive electrode (negative electrode) structure is folded using a long continuous folding separator sheet.
FIGS. 1 and 2 are diagrams illustrating an exemplary structure and a manufacturing process of the stack-folding type electrode assembly.
Referring to the drawings, as unit cells, C-type bicells 10, 13, and 14 of a negative electrode/separator/positive electrode/separator/negative electrode structure and A-type bicells 11 and 12 of a positive electrode/separator/negative electrode/separator/positive electrode structure are alternately stacked in a sequential order, and a folding separator sheet 20 is interposed between each stacked bicell. The folding separator sheet 20 has a unit length sufficient to surround the bicells, and is interposed between each stacked bicell in a structure that is folded inwards each unit length to continuously surround each bicell starting from the central bicell 10 to the outermost bicell 14. The folding separator sheet 20 is finished by heat welding or attaching, for example, an adhesive tape 25 at an end thereof.
The stack-folding type electrode assembly is made by, for example, arranging the bicells 10, 11, 12, 13, and 14 on the long folding separator sheet 20 and winding in a sequential order starting from one end 21 of the folding separator sheet 20.
In this instance, seeing an arrangement combination of the unit cells, the bicells 10, 11, 12, 13, and 14, the first bicell 10 and the second bicell 11 are spaced apart at a width interval corresponding to at least one bicell, and in the winding process, an outer surface of the first bicell 10 is completely covered with the folding separator sheet 20 and a bottom electrode (negative electrode, −) of the first bicell 10 comes into contact with a top electrode (positive electrode, +) of the second bicell 11. As a coating length of the folding separator sheet 20 increases during the sequential stack process by winding, the bicells 12, 13, and 14 subsequent to the second bicell 11 are arranged at a sequentially increasing interval therebetween in the winding direction. Also, because the bicells 10, 11, 12, 13, and 14 should be stacked such that a positive electrode and a negative electrode face each other at an interface during winding, the first bicell 10 is a bicell having a top electrode as a negative electrode, the second bicell 11 and the third bicell 12 is a bicell having a top electrode as a positive electrode, and the fourth bicell 13 and the fifth bicell 14 is a bicell having a top electrode as a negative electrode. That is, the bicells are mounted in an alternating arrangement by two units.
The stack-folding type electrode assembly makes up for the drawbacks of a jelly-roll type electrode assembly and a stack type electrode assembly, but when the number of bicell stacks increases for a high energy density, the number of folding increases, causing electrode assembly cell dimensional change/defect ratio increase and process time increase problems. As shown in FIG. 2, the type of an electrode placed on the folding separator sheet periodically changes, so a time loss occurs due to the cell type exchange (A-type bicell, C-type bicell) and the battery fabrication efficiency reduces.
Table 1 shows the number of electrodes with the increasing number of stacks in a stack-folding type electrode assembly such as one as shown in FIG. 1.
TABLE 1Number of stacks135791113. . .Number of electrodes391521273339. . .
Conventionally, a cell design involves increasing the number of electrodes by 6 with the increasing number of stacks as described above, so there is a limitation in changing the number of stacks for cell performance such as thickness, capacity, or resistance, resulting in low degree of freedom of design. Therefore, a stack-folding type electrode assembly with a reduced number of folding, high battery fabrication efficiency and a high degree of freedom of design and a method of manufacturing the same is required.